Spring brake actuator with spring force measurement

ABSTRACT

Measuring the force of a spring (28) in a spring brake actuator (10) comprising a housing (14) having an end wall (46), a spring plate (30) within the housing (14), the spring (28) being disposed between the spring plate (30) and the housing end wall (46) and a caging bolt (34) extending from the spring plate (30) through the housing end wall (46) and having a nut (44) thereon with a gauge between the caging bolt nut (44) and the housing (14). The gauge comprises a pressure-responsive element which detects the compressive force between the nut (44) and the housing (14) and generates a signal responsive thereto. An indicator (86) is coupled to the pressure-responsive element and visually displays the force on the spring (28) as a function of the compressive force signal. An embodiment of the gauge (50) comprises a body (52) having a bore (58), a piston (64) received within the bore (58) defining a chamber (74) between the piston (64) and the body (52) and a fluid (72) in the chamber (74).

BACKGROUND OF INVENTION

1. Field of the Invention

This invention relates to a spring brake actuator with a gauge formeasuring the spring force of a power spring in the actuator. In one ofits aspects, the invention relates to a method for measuring the forceof a spring in a spring brake actuator. In another of its aspects, theinvention relates to a gauge for measuring the force of a spring in aspring brake actuator.

2. State of the Prior Art

Spring brake actuators are in common use with air brake systems used ontrucks, buses, and towed vehicles. Such actuators normally include aservice brake actuator for applying and releasing the brakes in responseto delivery and exhaust of compressed air and a spring brake actuatordisposed in tandem with the service brake actuator. The spring brakeactuator uses spring force to operate the service brake actuator andapply the brakes when the air in a spring brake actuator chamber isreduced below a predetermined pressure. Air pressure may be reduced inthis chamber under the control of the operator to apply the brakes orautomatically as a result of failure of portions of the service brakeair system.

Typically, a barrel-shaped spring is used to store energy and to exertthe large force required for braking in the event of air pressurefailure. Air pressure in the chamber acting on a movable wall isemployed to compress the spring and maintain it in a retracted position.When the air is exhausted from the chamber, the spring acts on themovable wall, typically a diaphragm or a piston, and through anactuating rod to exert the spring force on the service actuator to applythe brakes.

It is desirable to measure the spring force of the spring to ensure thatit is operating within design parameters. However, disassembly of thespring brake actuator to remove the spring and test it outside theactuator does not measure its operating effectiveness on the vehicle.Moreover, many spring brake actuators now permanently enclose the powerspring, making removal of the spring for testing purposes impractical.

SUMMARY OF THE INVENTION

According to the invention, a gauge measures the force of a power springin a spring brake actuator comprising a housing having an end wall, aspring plate disposed within the housing, a power spring between thespring plate and housing end wall, and a caging bolt extending from thespring plate through the housing end wall and having a nut thereonexterior of the housing. The gauge comprises a pressure-responsiveelement adapted to be positioned between the caging bolt nut and thespring housing for generating a signal representative of the compressiveforce of the spring and an indicator coupled to the pressure-responsiveelement to visually display the force of the spring as a function of thepressure signal. Typically, the pressure-responsive element comprises afirst member adapted to receive the nut in abutting engagement and asecond member adjacent the first member and adapted to abut the housing.The first member is movable relative to the second member in response tocompressive force applied to the nut by the power spring acting againstthe spring plate. The gauge measures the compressive force between thenut and the spring housing so that the force applied by the power springcan be measured in situ. The invention provides quick and accurate insitu measurement of the force of the power spring. Also, measuring theforce of the spring with the brake installed in its operatingenvironment improves accuracy over measurements taken on brakes removedfrom the vehicle.

Preferably, the first and second members each have an aperture alignedto slidably receive the caging bolt. Also, the second member preferablycomprises a body having a bore, the first member comprises a pistonreceived within the bore, and the gauge further comprises apressure-responsive fluid between the piston and the body for resistingan applied compressive force and the pressure signal is a fluidpressure. The indicator is connected to the pressure-responsive fluidfor sensing and displaying the applied compressive force. Preferably,the indicator is a pressure sensor for measuring a pressure in the fluidand a display for displaying the spring force as a function of thepressure measurement. The pressure sensor can be calibrated to displaythe pressure reading in spring force units. Preferably, a threadedadjusting aperture is provided through the body into the chamber and ascrew is threaded into the adjusting aperture to adjust the volume ofthe chamber to calibrate the gauge.

Further according to the invention, a method for measuring the force ofa spring in a spring brake actuator comprising a housing, a spring platewithin the housing, a power spring between the spring plate and housingand a caging bolt extending from the spring plate exterior of thehousing and having a nut thereon comprises the steps of: detecting thepressure between the housing and the nut; generating a signalrepresentative of the detected pressure; and visually displaying anindication of the detected pressure as a function of the compressiveforce exerted by the power spring.

Further according to the invention, a spring brake actuator comprises ahousing having an end wall, a spring plate disposed within the housing,a power spring between the spring plate and the housing end wall, and athreaded caging bolt extending from the spring plate through the housingend wall, a nut on the caging bolt exterior of the housing and aforce-measuring gauge between the housing and the caging bolt nut tomeasure the force exerted by the power spring. The gauge comprises apressure-responsive element for detecting the compressive force betweenthe housing and the caging bolt nut and for generating a signalrepresentative of the compressive force, and an indicator coupled to thepressure-responsive element to visually display the force of the powerspring as a function of the representative signal. Thus, the forceapplied by the power spring can be measured in situ.

Preferably, the caging bolt further comprises at least one marking alongthe length thereof for positioning the nut thereon, wherein when the nutis aligned with the at least one marking and the spring is applying aforce to the spring plate, the spring will be at 50% of its maximumextension length or other predetermined length.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings in which:

FIG. 1 is a cross-sectional view of a spring brake actuator with aspring force gauge according to the invention;

FIG. 2 is an exploded view of a portion of the spring brake actuator ofFIG. 1 and the spring force gauge of FIG. 1; and

FIG. 3 is a cross-sectional view of a spring brake actuator with aspring force gauge according to a second embodiment of the invention.

DESCRIPTION

Referring now to the drawings and to FIG. 1 in particular, there isshown an air operated diaphragm spring brake 10 of the type disclosed inthe Ware U.S. Pat. No. 4,860,640 issued Aug. 29, 1989, incorporatedherein by reference. The air operated spring brake 10 comprises aservice brake housing 12 and spring housing 14 joined together intandem. A diaphragm 16 disposed within the service chamber 12 operatesagainst a pushrod 18 in a conventional manner to actuate the vehiclebraking system (not shown). A second diaphragm 20 divides the springhousing 14 into a spring chamber 22 and a pressure chamber 24. Anactuator rod 26 extends from the spring housing diaphragm 20 through thepressure chamber 24 and into the service housing 12. A powerfulbarrel-shaped spring 28 is disposed in the spring chamber 22 between thespring housing 14 and a pressure plate 30. The pressure plate 30 abutsthe spring housing diaphragm 20. When fully retracted, the spring 28applies a force of approximately 3,000 pounds against the pressure plate30.

During normal operation of the spring brake 10, a constant pressure isapplied to the pressure chamber 24, thereby holding the spring 28 in aretracted position as shown in FIG. 1. When pressure is released fromthe pressure chamber 24, the spring 28 extends and presses the actuatorrod 26 into the service housing 12.

Typically, a receptacle 32 is provided in association with the pressureplate 30 for receiving a caging bolt 34 so that the spring 28 can bemanually compressed and the actuator rod 26 retracted. Manual retractionwould be required, for example, when the brakes must be released, butthere had been a failure in the emergency brake air system. As moreclearly shown in FIG. 2, the caging bolt 34 comprises an elongatedthreaded rod 36 having a pair of radially extending projections 38 atone end thereof. The receptacle 32 has a keyed entry aperture 40 forreceiving the caging bolt projections 38. After the caging bolt 34 isfully received with the receptacle 32 it is rotated 90° so that theprojections 38 engage an annular shoulder 42 within the receptacle 32.Referring again also to FIG. 1, the caging bolt threaded rod 36 extendsout of the spring housing 14 through an end wall 46 thereof whereinwithdrawal of the caging bolt 34 will retract the pressure plate 30 tocompress the spring 28. Typically, a nut 44 is threaded onto the cagingbolt threaded rod 36 and torqued to a predetermined level to fullywithdraw or "cage" the power spring 28.

As shown in FIGS. 1 and 2, a gauge 50 according to the present inventionis interposed between the nut 44 and the spring housing 14 to measurethe compressive force of the power spring 28. Thus, the gauge 50 enablesone to test the compressive force of the spring 28 without removing thespring 28 from the spring housing 14, or otherwise disassembling thespring housing 14.

The gauge 50 comprises a discoidal body 52, the body 52 having an upperface 54 and a lower face 56. A first coaxial bore 58 extends partiallyinto the body 52 through its upper face 54. A second, smaller diameter,coaxial bore 60 extends into the body 52 from the lower face 56 tointersect the first bore 58, thereby forming an annular shoulder 62within the body 52 at the intersection of the first and second bores 58,60. A piston 64 is disposed within the body 52 and comprises a discoidalhead 66 disposed within the first bore 58, an annular collar 68extending axially from the head 66 and disposed within the second bore60. A coaxial bore 70 extends therethrough. A fluid chamber 74 definedin part by the piston head 66 and the annular shoulder 62 holds anincompressible fluid 72. An annular groove 76 about the annular collar68 receives an O-ring 78 and a second annular groove 80 about the pistonhead 66 receives a second O-ring 82 to seal the fluid 72 within thefluid chamber 74.

A threaded port 84 extends radially through the gauge body 52 into thefluid chamber 74 and receives a fluid pressure indicator 86. The cagingbolt 34 extends through the bore 70 with the nut 44 threaded onto thecaging bolt 34 in abutting engagement with an upper surface of thepiston head 66. Thus, force applied by the spring 28 to the pressureplate 30 is transmitted through the caging bolt 36 and the nut 44 to thepiston 64 thereby urging the piston 64 toward the annular shoulder 62,resisted only by the fluid 72. The resultant increased pressure in thefluid is displayed by the pressure indicator 86. It is preferable forthe indicator 86 to be calibrated to read the force of the spring 28 asopposed to the actual pressure in the fluid 72.

A snap ring 88 received within an annular groove 90 in the gauge body 52and adjacent the open end of the first bore 58 abuts an annular shoulder92 on the piston 64 to hold the piston 64 within the gauge body 52. Anadditional combined O-ring and plastic guide 94 may be provided betweenthe piston 64 and gauge body 52 adjacent the snap ring 88.

Provision is also made for calibrating the gauge 50. A threadedadjusting aperture 96 extends radially through the body 52 into thefluid chamber 74 and receives a threaded adjusting screw 98. Theadjusting screw 98 permits adjustment of the volume of the fluid chamber74 in order to calibrate the gauge 50. For instance, threading the screw94 into the adjusting aperture 92 reduces the volume of the fluidchamber and produces a correspondingly higher reading in the pressuregauge 86.

Industry standards for measuring the force of the compression spring 28call for reading the force measurement at 50 percent of the maximumstroke of the spring 28. Accordingly, the caging bolt 34 is preferablyprovided with a marking 100 on its threaded portion 36 which, whenaligned with the nut 44, corresponds to 50 percent of the maximum strokeof the spring 28. So that the caging bolt 34 can be employed with morethan one type or model of brake actuator, it may be provided withmultiple markings 100 corresponding to the various brake actuators forwhich it is contemplated. Preferably, the markings 100 will comprise aknurling in combination with a color coding. Also, the indicator 86 maybe calibrated with a "go," "no go" scale such as a red zone and greenzone (not shown) to readily indicate whether the spring 28 is applyingforce within design tolerances. Also, separate indicator faces may beprovided for separate types of brake actuators.

To read the force of a spring 28 in a given brake actuator 10, thecaging bolt 34 is first inserted into the receptacle 32 and properlyseated therein. The gauge 50 is then received over the caging bolt 34with the body lower face 56 abutting the spring housing 14 and the bore70 receiving the caging bolt threaded rod 36. The nut 44 is threadedonto the caging bolt threaded rod 36 and tightened to the predeterminedlocation indicated by the marking 100. To ease the burden of tighteningthe nut 44, pressure can be applied to the pressure chamber 24, therebyretracting the spring 28 so that the nut 44 can be freely threaded ontothe caging bolt 34 to its desired location. If pressurization of thepressure chamber 24 is impractical, the nut 44 can be threaded onto thecaging bolt 34 in the conventional manner. When the nut 44 is properlypositioned and the pressure is released from the pressure chamber 22,the indicator 86 will visually display the force applied by the spring28 at 50 percent of its maximum stroke. Preferably, the gauge 50 will becalibrated with the adjusting screw 98 prior to each use, with theadjusting screw 98 being adjusted so that the indicator 86 displays zeropressure when no force is applied to the piston 64.

Reasonable variation and modification are possible within the scope ofthe foregoing disclosure of the invention without departing from thespirit of the invention. For instance, the gauge 50 may take other formsthan an incompressible fluid gauge as illustrated in FIGS. 1 and 2. Forinstance, as shown in FIG. 3, wherein like parts in the previousembodiment are represented by like numerals, a piezoelectric crystal 110can be used to electronically detect the compressive force between thenut 44 and the housing 14 and to generate an electrical signalrepresentative thereof. A pair of wires 112 extend from thepiezoelectric crystal 110 to a gauge 114 that processes and displays theelectronic signals from the crystal 110 in a well known manner. Also,other mechanical means may be employed such as a plurality of stackedBelville springs or an elastomeric insert with the force readoutcomprising a change in the spatial relationship between the piston 64and body 52. It is to be understood that the description of theparticular embodiments contained herein is by way of illustration andnot limitation, and that the scope of the appended claim should beconstrued as broadly as the prior art will permit.

What is claimed is:
 1. A method for measuring the force of a spring in aspring brake actuator comprising a housing, a spring plate within thehousing, wherein the spring is positioned between the spring plate andthe housing, and a caging bolt extending from the spring plate exteriorof the housing and having a nut thereon, the method comprising the stepsof:detecting the compressive force between the caging bolt nut and thehousing; generating a signal representative of the detected compressiveforce; and visually displaying the force of the spring as a function ofthe compressive force signal.
 2. A method according to claim 1 whereinthe compressive force signal is a fluid pressure.
 3. A method accordingto claim 1 and further comprising the step of adjusting the nut on thecaging bolt so that the spring is at 50% of its maximum extension beforethe detecting step.
 4. A method according to claim 1 wherein thecompressive force signal is an electrical signal.
 5. In a spring brakeactuator comprising:a housing having an end wall; a spring platedisposed within the housing; a power spring between the spring plate andthe housing end wall; a threaded caging bolt extending from the springplate through the housing end wall; and a nut on the caging boltexterior of the housing, the improvement comprising:a force measuringgauge mounted between the housing end wall and the nut, the gaugecomprising:a pressure-responsive element between the caging bolt nut andthe housing end wall to detect the compressive force therebetween andfor generating a signal representative of said compressive force; and anindicator coupled to the pressure-responsive element for visuallydisplaying the force of the power spring as a function of thecompressive force signal, whereby the force exerted by the power springcan be measured in situ.
 6. A spring brake actuator according to claim 5wherein the pressure-responsive element comprises:a first memberabutting the nut; a second member adjacent to the first member andabutting the housing end wall, said first member being movable relativeto the second member in response to a force of the power spring actingon the nut through the spring plate and the caging bolt.
 7. A springbrake actuator according to claim 6 wherein the members each have anaperture, said apertures are aligned and the caging bolt extends throughthe apertures.
 8. A spring brake actuator according to claim 7 whereinthe second member comprises a body having a bore, the first membercomprises a piston received within the bore and defining a chamberbetween the piston and the body, and the pressure-responsive elementfurther comprises a pressure-responsive fluid in the chamber forresisting the compressive force.
 9. A spring brake actuator according toclaim 8 wherein the signal generated by the pressure-responsive elementis fluid pressure which is a function of the fluid pressure in thechamber.
 10. A spring brake actuator according to claim 9 wherein theindicator is calibrated to display a reading in spring force units. 11.A spring brake actuator according to claim 8 and further comprising athreaded adjusting aperture through the body into the chamber and ascrew threaded into the adjusting aperture whereby the volume of thechamber can be adjusted to calibrate the gauge.
 12. A spring brakeactuator according to claim 5 wherein the power spring further has amaximum extension length and the caging bolt further comprises a markingfor positioning the nut thereon, wherein when the nut is aligned withthe marking and the spring is applying a force to the gauge, the springwill be at 50% of its maximum extension length.
 13. A spring brakeactuator according to claim 5 wherein the signal generated by thepressure-responsive element is a fluid pressure.
 14. A spring brakeactuator according to claim 5 wherein the signal generated by thepressure-responsive element is an electrical signal.